FileTransferProtocol
File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and uses separate control and data connections between the client and the server. FTP users may authenticate themselves using a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that hides (encrypts) the username and password, and encrypts the content, FTP is oftensecured with SSL/TLS ("FTPS"). SSH File Transfer Protocol ("SFTP") is sometimes also used instead, but is technologically different. The first FTP client applications were command-line applications developed before operating systems had graphical user interfaces, and are still shipped with most Windows, Unix, and Linux operating systems. Dozens of FTP clients and automation utilities have since been developed for desktops, servers, mobile devices, and hardware, and FTP has been incorporated into hundreds of productivity applications, such as Web page editors. An Overview of the File Transfer Protocol The File Transfer Protocol (FTP) was one of the first efforts to create a standard means of exchanging files over a TCP/IP network, so the FTP has been around since the 1970's. The FTP was designed with as much flexibility as possible, so it could be used over networks other than TCP/IP, as well as being engineered to have the capability with exchanging files with a broad variety of machines. The base specification is RFC 959 and is dated October 1985. There are some additional RFCs relating to FTP, but it should be noted that even as of this writing (December 2001) that most of the new additions are not in widespread use. The purpose of this document is to provide general information about how the protocol works without getting into too many technical details. RFC 959 should be consulted for details on the protocol. RFC 959 Introduction: The objectives of FTP are 1) to promote sharing of files (computer programs and/or data), 2) to encourage indirect or implicit (via programs) use of remote computers, 3) to shield a user from variations in file storage systems among hosts, and 4) to transfer data reliably and efficiently. FTP, though usable directly by a user at a terminal, is designed mainly for use by programs. OVERVIEW In this section, the history, the terminology, and the FTP model are discussed. The terms defined in this section are only those that have special significance in FTP. Some of the terminology is very specific to the FTP model; some readers may wish to turn to the section on the FTP model while reviewing the terminology. History The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as RFC 114 on 16 April 1971. Until 1980, FTP ran on NCP, the predecessor of TCP/IP.The protocol was later replaced by a TCP/IP version, RFC 765 (June 1980) and RFC 959 (October 1985), the current specification. Several proposed standards amend RFC 959, for example RFC 2228 (June 1997) proposes security extensions and RFC 2428 (September 1998) adds support for IPv6 and defines a new type of passive mode.[4] FTP Development and Standardization The developers of early applications conceptually divided methods of network use into two categories: direct and indirect. Direct network applications let a user access a remote host and use it as if it were local, creating the illusion that the network doesn't even exist (or at least, minimizing the importance of distance). Indirect network use meant getting resources from a remote host and using them on the local system, then transferring them back. These two methods of use became the models for the first two formalized TCP/IP networking applications: Telnet for direct access and the File Transfer Protocol (FTP) for indirect network use. The first FTP standard was RFC 114, published in April 1971, before TCP and IP even existed. This standard defined the basic commands of the protocol and the formal means by which devises communicate using it. At this time the predecessor of TCP (called simply the Network Control Protocol or NCP) was used for conveying network traffic. There was no Internet back then. Its precursor, the ARPAnet, was tiny, consisting of only a small group of development computers. Protocol overview The server responds over the control connection with three-digit status codes in ASCII with an optional text message. For example "200" (or "200 OK") means that the last command was successful. The numbers represent the code for the response and the optional text represents a human-readable explanation or request (e.g. ).An ongoing transfer of file data over the data connection can be aborted using an interrupt message sent over the control connection.FTP may run in active or passive mode, which determines how the data connection is established. In active mode, the client creates a TCP control connection. In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used. In this mode, the client uses the control connection to send a PASV command to the server and then receives a server IP address and server port number from the server, which the client then uses to open a data connection from an arbitrary client port to the server IP address and server port number received.Both modes were updated in September 1998 to support IPv6. Further changes were introduced to the passive mode at that time, updating it to extended passive mode. While transferring data over the network, four data representations can be used: *ASCII mode: used for text. Data is converted, if needed, from the sending host's character representation to "8-bit ASCII" before transmission, and (again, if necessary) to the receiving host's character representation. As a consequence, this mode is inappropriate for files that contain data other than plain text. *Image mode (commonly called Binary mode): the sending machine sends each file byte for byte, and the recipient stores the bytestream as it receives it. (Image mode support has been recommended for all implementations of FTP). *EBCDIC mode: use for plain text between hosts using the EBCDIC character set. This mode is otherwise like ASCII mode. *Local mode: Allows two computers with identical setups to send data in a proprietary format without the need to convert it to ASCII For text files, different format control and record structure options are provided. These features were designed to facilitate files containing Telnet or ASA Data transfer can be done in any of three modes: *Stream mode: Data is sent as a continuous stream, relieving FTP from doing any processing. Rather, all processing is left up to TCP. No End-of-file indicator is needed, unless the data is divided into records. *Block mode: FTP breaks the data into several blocks (block header, byte count, and data field) and then passes it on to TCP. *Compressed mode: Data is compressed using a single algorithm (usually run-length encoding). Security FTP was not designed to be a secure protocol—especially by today's standards—and has many security weaknesses. In May 1999, the authors of RFC 2577 listed a vulnerability to the following problems: · Brute force attacks · Bounce attacks · Packet capture (sniffing) · Port stealing · Spoof attacks · Username protection FTP is not able to encrypt its traffic; all transmissions are in clear text, and usernames, passwords, commands and data can be easily read by anyone able to perform packet capture (sniffing) on the network.[2][16] This problem is common to many of the Internet Protocol specifications (such as SMTP, Telnet, POP and IMAP) that were designed prior to the creation of encryption mechanisms such as TLS or SSL. A common solution to this problem is to use the "secure", TLS-protected versions of the insecure protocols (e.g. FTPS for FTP, TelnetS for Telnet, etc.) or a different, more secure protocol that can handle the job, such as the SFTP/SCP tools included with most implementations of the Secure Shell protocol. Secure FTP There are several methods of securely transferring files that have been called "Secure FTP" at one point or another. FTPS Explicit FTPS is an extension to the FTP standard that allows clients to request that the FTP session be encrypted. This is done by sending the "AUTH TLS" command. The server has the option of allowing or denying connections that do not request TLS. This protocol extension is defined in the proposed standard: RFC 4217. Implicit FTPS is a deprecated standard for FTP that required the use of a SSL or TLS connection. It was specified to use different ports than plain FTP. SFTP SFTP, the "SSH File Transfer Protocol", is not related to FTP except that it also transfers files and has a similar command set for users. SFTP, or secure FTP, is a program that uses Secure Shell (SSH) to transfer files. Unlike standard FTP, it encrypts both commands and data, preventing passwords and sensitive information from being transmitted openly over the network. It is functionally similar to FTP, but because it uses a different protocol, standard FTP clients cannot be used to talk to an SFTP server, nor can one connect to an FTP server with a client that supports only SFTP. FTP over SSH (not SFTP) FTP over SSH (not SFTP) refers to the practice of tunneling a normal FTP session over an SSH connection.[16] Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. With many SSH clients, attempting to set up a tunnel for the control channel (the initial client-to-server connection on port 21) will protect only that channel; when data is transferred, the FTP software at either end will set up new TCP connections (data channels), which bypass the SSH connection and thus have no confidentiality or integrity protection, etc. Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, to monitor and rewrite FTP control channel messages and autonomously open new packet forwardings for FTP data channels. Software packages that support this mode include: · Tectia ConnectSecure (Win/Linux/Unix) of SSH Communications Security's software suite · Tectia Server for IBM z/OS of SSH Communications Security's software suite · FONC (the GPL licensed) · Co:Z FTPSSH Proxy Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol. FTP reply codes Main article: List of FTP server return codesBelow is a summary of the reply codes that may be returned by an FTP server. These codes have been standardized in RFC 959 by the IETF. As stated earlier in this article, the reply code is a three-digit value. The first digit is used to indicate one of three possible outcomes—success, failure or to indicate an error or incomplete reply: *2yz – Success reply *4yz or 5yz – Failure Reply *1yz or 3yz – Error or Incomplete reply The second digit defines the kind of error: *x0z – Syntax. These replies refer to syntax errors. *x1z – Information. Replies to requests for information. *x2z – Connections. Replies referring to the control and data connections. *x3z – Authentication and accounting. Replies for the login process and accounting procedures. *x4z – Not defined. *x5z – File system. These replies relay status codes from the server file system. The third digit of the reply code is used to provide additional detail for each of the categories defined by the second digit. The advantages and disadvantages of FTP clients Here are some advantages to using an FTP client. Allows you to transfer multiple files as well as directories The ability to resume a transfer is the connection is lost The ablilty to add items to a “queue” to be uploaded/downloaded Many FTP clients have the ability to schedule transfers No size limitation on single transfers (browsers only allow up to 2 GB) Many clients have scripting capabilities through command line Most clients have a synchronizing utility Faster transfers then HTTP Those are just a few of the advantages of using an FTP client. While the clients help make transfers easier they are not without their drawbacks. Here are some of the disadvantages of using and FTP client. Usernames, passwords and files are sent in clear text Filtering active FTP connections is difficult on your local machine (passive is preferred) Servers can be spoofed to send data to a random port on an unintended computer In those cases you can see where using the web interface would eliminate these issues. Both FTP clients and the FTP site have their advantages and disadvantages. Fortunately you are able to use them hand in hand depending on your specific needs. As browsers continue to advance more and more of these features will be worked in. In the future the need for a third party client may be unnecessary, but for now they are really nice to have.